Port-Channel and spanning-tree cost command

[sebastien3]

Hello,

I configured a Port-Channel in trunk mode with two physical ports on my switch.

In the PO5 if I add spanning-tree vlan 10,15 cost 100, this command is not duplicated on the member interfaces of the PO1 (Te1/1/1 and Te1/1/2).

interface Port-channel1
switchport
switchport trunk allowed vlan 10-15
switchport mode trunk
spanning-tree vlan 10,15 cost 100
!
interface TenGigabitEthernet1/1/1
description member of PO1
switchport
switchport trunk allowed vlan 10-15
switchport mode trunk
!
interface TenGigabitEthernet1/1/2
description member of PO1
switchport
switchport trunk allowed vlan 10-15
switchport mode trunk

Is this normal ?

Thanks

[Kasun Bandara]

@sebastien3 hi normally, you need to configure settings on Port channel interface only. but in your question, you are mentioned about PO5 ?

[sebastien3]

Mistake... PO1 not PO5.

[paul driver]

Hello
Spanning-tree sees the port-channel as a single interface as such it will calculate the stp cost based on the multiple interfaces allocated to it, So if you manually apply an stp cost to the PC it should not append that cost its related physical interfaces.

[sebastien3]

So I need to enable this command on member interfaces (TE1/1/1 and TE1/1/2) in addition to port-channel1 ?

[Flavio Miranda]

Hi @sebastien3 

 This is the expected behavior for Port-channel and spenning-tree and you dont need to add the config to the member interface. You can check on this Lab for CCNP for Cisco Academy as reference.

 The spanning-tree sees the port-channel as one interface therefore will not treat each interface individually but as part of the channel.

"Verify the EtherChannel has formed by examining the output of the show etherchannel summary command. Also check the spanning tree status. You will see that the two interfaces are no longer referenced by Spanning Tree, but the port-channel is. Because there is only one (logical) trunk between D1 and D1, there are no Spanning Tree blocked ports."

[M02@rt37]

Hello @sebastien3,

When you configure a Port-Channel, the logical interface (Port-Channel interface) acts as a single aggregated link that represents the bundled member interfaces. STP treats the Port-Channel as a single entity, and STP settings, such as the cost and VLAN assignments, are applied to the entire Port-Channel, not to its individual member interfaces.

This allows the Port-Channel to be treated as a single link for STP purposes, providing loop prevention and redundancy without the need to configure STP settings on each member interface separately. The individual member interfaces are essentially hidden from STP, and all STP-related decisions are made based on the logical Port-Channel.

[sebastien3]

Hello M02@rt37 

So this is normal behavior. SPT cost must be configured on the Port-Channel and not on the physical interfaces... No bug or mishandling on the configuration.

[M02@rt37 · Accepted solution]

@sebastien3,

This is normal and expected behavior.